Modeling of
BCN V2.0
Jinjing Jiang and Raj Jain
Washington University in Saint Louis
Saint Louis, MO 63130
Jain@wustl.edu
IEEE 802.1 Congestion Group Meeting, San Diego, July 19, 2006
These slides are available on-line at:
http://www.cse.wustl.edu/~jain/ieee/bcn607.htm
IEEE 802.1 Meeting July 19, 2006
1
Raj Jain
Overview
Goal: Present new results since Denver/March 2006
 BCN Mechanism: Quick Review
 Action Items from Denver meeting
 New Analytical Results
 New Simulation Results

IEEE 802.1 Meeting July 19, 2006
2
Raj Jain
BCN Mechanism
Rate Regulator
Congestion Point
BCN


Qsc Qeq
Backward Congestion Notification - Closed loop feedback
 Detection: Monitor the buffer utilization at possible
congestion point (Core Switch, etc)
 Signaling: Generate proper BCN message based the status
and variation of queue buffer
 Reaction: At the source side, adjust the rate limiter setting
according to the received BCN messages
 Additive Increase Multiplicative Decrease (AIMD)
Ref: new-bergamasco-backward-congestion-notification-0505.pdf
IEEE 802.1 Meeting July 19, 2006
4
Raj Jain
Parameters for BCN
Rate Regulator
Congestion Point
Arrivals
Qsc Qeq
Departures
BCN


Key Parameters
 Threshold for buffer:
 Qeq (Equilibrium),
 Qsc (Severe Congestion),
Queue Variation : Qoff, Qdelta
 Queue is sampled randomly with probability Pm = 0.01
 Qlen (current length)
 Qoff = Qeq-Qlen, [-Qeq, +Qeq]
 Qdelta = #pktArrival-#pktDeparture, [-2Qeq, +2Qeq]
IEEE 802.1 Meeting July 19, 2006
5
Raj Jain
AIMD Algorithm





Source Rate R
Feedback
 Fb = (Qoff - W×Qdelta)
Additive Increase (Fb > 0)
 R = R + Gi×Fb×Ru
Multiplicative Decrease (Fb < 0)
 R = R×(1 - Gd×Fb)
Parameters used in AIMD:
1. Derivative weight W
2. Additive Increase gain Gi,
3. Multiplicative Decease Gain Gd,
4. Rate Unit Ru
IEEE 802.1 Meeting July 19, 2006
6
Raj Jain
Summary of Results (From March Meeting)
BCN V2 simulation validate Cisco’s results on
throughput
 Time to Fairness and oscillation trade-off needs to be
studied further
 Parameter setting needs more work
Need to modify formula so that parameters are
dimensionless
 Need to simulate more configurations:
asymmetric, larger bandwidth delay, and multibottleneck cases

IEEE 802.1 Meeting July 19, 2006
7
Raj Jain
Issues to be Studied (From March Meeting)
1. Fix the dimensioning problem
2. Asymmetric Topology
3. Multi-bottleneck case
4. Larger/smaller Bandwidth×Delay product networks
5. Bursty Traffic
6. Non-TCP traffic
7. Interaction with TCP congestion mechanism
8. Effect of BCN/Tag messages getting lost
We present results on the first 6 issues + an analytical
model + proportional fairness
IEEE 802.1 Meeting July 19, 2006
8
Raj Jain
Topics for Today
1.
2.
3.
4.
5.
6.
7.
8.
9.
Analytical Model
Simulation Study: Convergence
Dimensioning Problem
Asymmetric Topology and Multiple Congestion
Points
Max-min vs Proportional Fairness
Mixed TCP and UDP Traffic
Bursty Traffic
Other Issues
Bandwidth*Delay Product
IEEE 802.1 Meeting July 19, 2006
9
Raj Jain
1. Analytical Model

See Wash U technical report, which will be posted shortly
IEEE 802.1 Meeting July 19, 2006
10
Raj Jain
Analytical Model Results
Rate of Convergence:
Conclusions:
 Increasing Gi, Gd and Ru will always increase the rate of
convergence
 Feedback Delay= Sampling+Propagation+Switching+Reaction
Sampling Delay =
= Pkt Size/(input rate*Sampling P)
 Bandwidth*delay (delay=Propagation and switch delays) may
not be related to the operation of the BCN mechanism
 Sampling probability Pm is the key parameter.
Should be carefully selected considering current input rate ri
and packet size Sp
IEEE 802.1 Meeting July 19, 2006
11
Raj Jain
2. Simulation Study: Convergence






Goal: To find optimal parameters for least oscillation
Topology:
Two Configurations: All links 1 Gbps or All links 10 Gbps
Two Values for Rate Increment Ru
Two values for Sampling Probability Pm
A 22 Full factorial experimental design
[Art of Computer Systems Performance Analysis]
IEEE 802.1 Meeting July 19, 2006
12
Raj Jain
Simulation on Convergence – 1Gbps Link
Ru = 8 Mbps Pm = 0.01
Ru =0.8 Mbps Pm = 0.01
Ru = 8 Mbps Pm = 0.1
Ru = 0.8 Mbps Pm = 0.1
Best
IEEE 802.1 Meeting July 19, 2006
13
Raj Jain
Simulation on Convergence - 10Gbps Link
Ru = 8 Mbps Pm = 0.01
Ru =4 Mbps Pm = 0.01
Ru = 8 Mbps Pm = 0.05
Ru = 4 Mbps Pm = 0.05
Best
IEEE 802.1 Meeting July 19, 2006
14
Raj Jain
Simulation on Convergence: Conclusions
Large Pm, small Ru make oscillations smaller
in both cases
 Larger Pm=> excessive signaling overhead
 Small Ru=> long time to converge
 Parameters depend upon bottleneck link speed

IEEE 802.1 Meeting July 19, 2006
15
Raj Jain
3. Dimensioning Problem
1 Gbps Link and 10 Gbps Link
 Same Pm and Ru leads to instability
 Sources need to know the bottleneck link capacity
Need to add bottleneck rate to the BCN message.
 Current BCN mechanism sets 5 Gbps as the initial rate
for rate limiter. If congested link capacity (1 Gbps) is
not known at the source, it takes long time for the
sources to decrease their rates to less than 1 Gbps.
 The rate increase unit Ru should be set as C/N for
some N. If not, there are large oscillations

IEEE 802.1 Meeting July 19, 2006
16
Raj Jain
4. Asymmetric Topology and Multiple
Congestion Points
Topology: Only one link is 1Gbps, others are all
10Gbps
IEEE 802.1 Meeting July 19, 2006
17
Raj Jain
A Simulation Result on BCN and the
Enhanced Version
Not Stable
Stable with
oscillations
(a, b) Pm=0.01, Ru=8Mbps for all links (c, d) Pm=0.01, Ru=8Mbps for 10Gbps
links, Pm=0.1, Ru=0.8Mbps for 1Gbps link
IEEE 802.1 Meeting July 19, 2006
18
Raj Jain
5. Max-min vs Proportional Fairness

Max-Min Fairness: Assumes linear utility of data rate
Maximize the minimum allocation w/o exceeding the capacity
maximize

Proportional Fairness: Data rate has a log utility
Maximize the sum of the logs w/o exceeding the capacity
maximize
IEEE 802.1 Meeting July 19, 2006
19
Raj Jain
Simulations for Fairness

Simulation using Parking Lot Topology

Max-min fairness: R01 =…=R04 =R1=R2=C/5
Proportional fairness: R01=…=R04 =C/6; R1=R2=C/3

IEEE 802.1 Meeting July 19, 2006
20
Raj Jain
Simulation on Fairness

Simulation results for Parking Lot topology(Gbps):
 R01=1.4643, R02=1.4532
 R03=1.5430, R04=1.7291
 R1=3.0795, R2=3.0185
 2(R1+R2)/(R01+…+R04)=1.97≈2
IEEE 802.1 Meeting July 19, 2006
21
Raj Jain
6. Mixed TCP and UDP Traffic
Stable
IEEE 802.1 Meeting July 19, 2006
22
Raj Jain
Mixed TCP and UDP Traffic: Results
TCP average throughput:1.16 Gbps
UDP average throughput:1.34 Gbps
No significant performance difference compared with
TCP-only workload
 Since rate limiter is implemented at the sources, UDP
rate is also controlled
 UDP has slightly higher throughput than TCP
 TCP has its own congestion control mechanism, rate
limiter’s rate is the peak rate is can achieve.

IEEE 802.1 Meeting July 19, 2006
23
Raj Jain
7. Bursty Traffic



If the burst period is much longer than the settling time of the
system, the system is still stable.
If not, the system tends to be unstable.
Settling time  4 ms for the above simulations
UDP is bursty with Pareto distribution for Burst size, Topology
for mixed traffic
IEEE 802.1 Meeting July 19, 2006
24
Raj Jain
8. Other Issues



BCN(0,0) is sent when the queue is severely congested. It asks
source to stop and restart at 1/100th of the link capacity after a
some random interval.
This leads to low throughput.
In the original BCN message, sending back Qoff=0, Qdelta=0 to
indicate the severe congestion, which may cause low link
utilization
 Qoff=0, Qdelta=0 is very likely when the queue operates at
the equilibrium
 Our results in March presentation have larger oscillation is
purely because of the different use of BCN(0,0) message
IEEE 802.1 Meeting July 19, 2006
25
Raj Jain
9. Bandwidth*Delay Product

In this simulation, the symmetric topology is used, propagation delay is 9.5 us
(originally it is 0.5 us), which to some extent, we use this to simulate 7 hops network
from the source to the congested switch
The queue is stable.
As aforementioned,
propagation delay
have small effect on
the feedback delay
IEEE 802.1 Meeting July 19, 2006
26
Raj Jain
Summary
1.
2.
3.
4.
5.
We have developed an analytical model of BCN that allows us to study the
effect of various parameters on convergence and stability
BCN achieves Proportional Fairness (vs max-min fairness)
Need to feedback bottleneck capacity in the BCN message
Optimal parameters depend upon the bottleneck capacity
Performance of BCN (including bottleneck rate feedback):
1. TCP and UDP mixed traffic
2. Performance with Multiple Congestion Point
3. Bursty Traffic
4. Different Bandwidth*Delay product networks
IEEE 802.1 Meeting July 19, 2006
27
Raj Jain
Thank You!
IEEE 802.1 Meeting July 19, 2006
28
Raj Jain